1. Field of the Invention
This invention is related to a metal hydride storage canister, particularly a canister for use with a hydrogen fuel cell. The canister enhances thermal conductivity and provides space required by the expansion of the metal hydride. This invention further discloses the manufacture of the storage canister.
2. Description of the Related Art
With the rapid growth of human civilization the consumption of traditional energy sources, such as coal, oil and natural gas, increases rapidly. This results in serious pollution to the global environment and causes various environmental problems such as global warming and acid rain. It is now recognized that the existing natural energy resources are limited. Therefore, if the present rate of energy consumption continues, all existing natural energy sources will be exhausted in the near future. Accordingly, many developed countries are dedicated to the research and development of new and alternative energy sources. The fuel cell is one of the most important and reasonably priced energy sources. Compared with traditional internal combustion engines, the fuel cell has many advantages such as high-energy conversion efficiency, clean exhaust, low noise, and no consumption of traditional gasoline.
In brief, a fuel cell is an electrical power generation device powered by the electrochemical reaction of hydrogen and oxygen. Basically, the reaction is a reverse reaction of the electrolysis of water, which converts the chemical energy into electrical energy. The basic structure of a fuel cell, for example, a proton exchange membrane fuel cell, comprising a plurality of cell units. Each cell unit contains a proton exchange membrane (PEM) at the middle, with the two sides thereof provided with a layer of catalyst. Each of the two outsides of the catalyst is further provided with a gas diffusion layer (GDL). An anode plate and a cathode plate are further provided at the outermost sides adjacent to the GDL. After all of the above elements are combined together, a cell unit is formed.
For the practical application of a fuel cell, a plurality of the above cell units are stacked and serially connected to form a cell stack for providing sufficient power, The cell stack is positioned between two end plates provided at the longitudinal, opposing ends of the cell stack. A plurality of tie rods pass through a peripheral region of each end plate for positioning the cell stack between the two end plates.
While performing the aforesaid reverse reaction of the electrolysis of water, in order to convert the chemical energy into electrical energy, the cell stack must be maintained under a consistent pressure range. This ensures that the reverse reaction of the electrolysis of water is performed under the optimum pressure condition to enhance the conversion efficiency of the chemical energy into electrical energy.
One known measure of storing hydrogen is to use the so-called metal hydride. Metal hydride is able to discharge hydrogen at a pressure corresponding to the temperature that it experiences; the process of releasing hydrogen is an endothermic reaction. When the hydrogen stored within the metal hydride has been completely exhausted, pure hydrogen can be re-charged back to the metal hydride; the process of charging hydrogen is an exothermic reaction. The temperature that metal hydride experiences is positively proportional to the pressure of the hydrogen to be discharged from the metal hydride. Such a proportional relationship may vary among metal hydrides furnished by different suppliers.
Due to the highly combustive characteristic of hydrogen, an easy and convenient method for pre-storing hydrogen within a specific container, and for releasing hydrogen as required for performing the above reverse reaction, is needed. The commonly known storage container is mostly a metal container capable of sustaining a pre-determined pressure and temperature. The container is sealed after a pre-determined amount of metal hydride is loaded therein.
Since the volume of metal hydride increases 5 to 20% after being charged with hydrogen, excessive space must be reserved when loading the metal hydride into the container to provide the space required by expansion of the metal hydride. Expansion of the metal hydride will easily cause compaction of the metal hydride when it is placed within a mutual space in conventional containers. The exothermic reaction during the process of charging hydrogen may cause an increment in temperature thereby, reducing the rate of charging hydrogen, such that the process must rely on the container surface to release the thermal energy to reduce the temperature. On the contrary, the endothermic reaction during the process of discharging hydrogen also relies on the container surface to absorb heat to efficiently increase rate of heat transfer. The mechanism for enhancing the rate of heat transfer is essential because metal hydride has a relatively low thermal conductivity. Furthermore, an easy passageway is needed for guiding the hydrogen discharged from the metal hydride to an outlet of the container.
It should also be noted that aside from fuel cells, the metal hydride storage canister according to this invention could also be adapted in other applications, such as hydrogen driven pumps, and hydrogen driven air-conditioners.